To begin:
Bog standard Beowulf netinstall + MPD and deps. No bloat, no monitor, just a dumpster-special Pentium 4 SSF desktop, 1GB RAM, and an old Xonar DS I had laying around.

MPD generally works fine, so long as I use the software mixer or an ALSA softvol plugin. But software volume control over the range I need sounds like arse, and the card in question has a nice hardware mixer I would very much like to use.
Like is probably an understatement here TBH, the thing is hooked directly to an old-school dumb-as-rocks 350WPC (real, continuous RMS Watts mind you) power amplifier. That means no volume knob, nada, nothing but a power switch, 2 inputs and 2 outputs. I want that soundcard mixer working.

A random selection of other CLI media players work just fine with the hardware mixer, as does mopidy's alsamixer plugin. But it's mopidy, and that means dog-slow python-hell.

Here's a snip from a 0-100% volume ramp with mpc and the corresponding output from amixer:

volume: 60%   repeat: off   random: off   single: off   consume: off
  Front Left: Playback 232 [81%] [-11.50dB] [on]
  Front Right: Playback 232 [81%] [-11.50dB] [on]
volume: 62%   repeat: off   random: off   single: off   consume: off
  Front Left: Playback 233 [82%] [-11.00dB] [on]
  Front Right: Playback 233 [82%] [-11.00dB] [on]
volume: 62%   repeat: off   random: off   single: off   consume: off
  Front Left: Playback 233 [82%] [-11.00dB] [on]
  Front Right: Playback 233 [82%] [-11.00dB] [on]
volume: 63%   repeat: off   random: off   single: off   consume: off
  Front Left: Playback 234 [82%] [-10.50dB] [on]
  Front Right: Playback 234 [82%] [-10.50dB] [on]
volume: 65%   repeat: off   random: off   single: off   consume: off
  Front Left: Playback 235 [83%] [-10.00dB] [on]
  Front Right: Playback 235 [83%] [-10.00dB] [on]
volume: 65%   repeat: off   random: off   single: off   consume: off
  Front Left: Playback 235 [83%] [-10.00dB] [on]
  Front Right: Playback 235 [83%] [-10.00dB] [on]
volume: n/a   repeat: off   random: off   single: off   consume: off
  Front Left: Playback 2 [-111%] [-126.50dB] [on]
  Front Right: Playback 2 [-111%] [-126.50dB] [on]
volume: 68%   repeat: off   random: off   single: off   consume: off
  Front Left: Playback 237 [85%] [-9.00dB] [on]
  Front Right: Playback 237 [85%] [-9.00dB] [on]
volume: 68%   repeat: off   random: off   single: off   consume: off
  Front Left: Playback 237 [85%] [-9.00dB] [on]
  Front Right: Playback 237 [85%] [-9.00dB] [on]
volume: 69%   repeat: off   random: off   single: off   consume: off
  Front Left: Playback 238 [86%] [-8.50dB] [on]
  Front Right: Playback 238 [86%] [-8.50dB] [on]

And here's the full run.

See that glitch in the middle there where 84% takes a random holiday? That's potentially "I have no windows any more" levels of not-good, and it makes most MPD clients have kittens.

There's nothing unusual in my mpd.conf, and nothing weird in my /etc/asound.conf or ~/.asoundrc either, as those files are absent.
amixer output looks like this, again nothing odd except perhaps that the "Master" control is multichannel.

The Debian/Devuan MPD build is so fossilised upstream won't speak of it, so I went as far as to compile the latest (0.22.6) from source, with identical results.
I'm now back with 0.21.5 from the repos, and as the instructions on that one amount to "too old, go bug Debian about it", here I am.

Anyone have any ideas that aren't "Use softvol" or "Use mopidy"?